Observational Studies of Massive Star Forming Regions

Lasrado, Akhil (2022) Observational Studies of Massive Star Forming Regions. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Akhil Lasrado (17MS073)) 17MS073_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

Understanding the origins of stars in the Universe is a challenging task, and while a great deal of the stages of star formation at the low-mass end have been detailed, a conclusive theory for their high mass counterparts remains unknown. This is in great part due to the many observational challenges associated with studying regions conducive to the formation of massive stars (1We will use the terms massive stars and high-mass star interchangeably throughout this report, which refers to a star of mass roughly > 8M⊙), which includes the high degree to which molecular gas and astrophysical dust obscure the visible wavelength view of these regions. Overcoming this limitation requires us to turn to observations in the infrared regime, which subsequently suffer from a reduction in resolution. Nevertheless, the use of data from instruments aboard observatories such as Herschel, Spitzer, APEX, Planck, and ALMA have allowed a detailed study into star-forming regions over the past decade or so. In the context of high-mass star formation, one of the foremost paradigms put forward involves competitive accretion of material in star-forming clouds of molecular hydrogen. The distribution of mass in the cloud gives rise to a gravitational potential landscape, and material will be favourably transported to the minima of this landscape. Newly forming stars located at these minima are understood to benefit from the excess accretion rates, and the resulting conditions are understood to favour massive star formation. A particular class of star-forming regions where competitive accretion is most apparent, is the so-called class of “hub-filament” systems (HFSs). Material within molecular clouds has been long understood to organize itself into filamentary structures. These filaments form highly dense “hubs” upon intersection, and these hubs are the putative sites of massive star formation since material from the entire molecular cloud is transported towards them, akin to the scenario stated by competitive accretion. The significant insight into massive star formation that can be obtained from studying these configurations have resulted in them being an active topic in star formation research. In this project, we perform a comprehensive study of observations in the infrared continuum regime of a hub-filament system referred to as HF09729. We ensure first that the selected HFS is correctly identified to occur as an intersection of filaments through the use of distance estimated from an existing molecular line survey catalogue of molecular clouds. Following this we take a look into the distribution of young stellar objects, and star forming clumps from mid- and far-infrared source catalogues. Looking into detailed far-infrared emission in the region primarily obtained by Herschel, we confirm the presence of filamentary structures as dark clouds in the infrared, signifying elevated levels of gas and dust. This leads into an exploration into various methods of the derivation of molecular hydrogen column density, and dust temperature maps of the region. We first perform a traditional pixel-by-pixel multi-wavelength fit of the dust modelled by a modified blackbody function. This gives us preliminary insight into the gas density levels at hierarchical stages of cloud collapse. The relatively long wavelength of infrared observations lead to a proportional decrease in resolution, and over the past decade, particular methods have been incorporated in star formation studies to obtain higher resolution column density maps. In this project we study two such methods, namely a spatial filtering-based approach, and PPMAP, a Bayesian procedure which outputs temperature-differentiated column density maps which allows for the distinguishing of dusty features along the line-of-sight, and at different temperature intervals. Using information from the latter, we can estimate the masses of various structures in the region, which we use to obtain a star-forming efficiency for the molecular cloud of about ≈ 0.3%. This is consistent with the picture we build for the observed region as a quiescent star-forming cloud, containing a strong young massive star candidate.

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